Paramagnetic methods, which are based on the fact that oxygen molecules are paramagnetic based on their permanent magnetic dipole moment, whereas most other gases are diamagnetic, are frequently used to determine the oxygen concentration in gases. It is generally known that the heat conductivity changes in paramagnetic gases (for example, O2 and NO) under the effect of magnetic fields. The cause of this behavior is obviously the fact that paramagnetic gases have a permanent magnetic moment, but this is not normally manifested towards the outside because of the thermal motion of the molecules. However, a sufficiently strong external magnetic field ensures that the magnetic dipole moments of the individual molecules are aligned. This brings about, on the one hand, a change in susceptibility, which leads to an increase in magnetic flux, and, on the other hand, a certain molecular arrangement becomes established in the gas, as a result of which the possibility of transmitting heat energy to adjacent molecules by shocks is limited. Consequently, the heat conductivity of the gas changes to a small extent.
In a prior-art measuring device, which is based on this phenomenon, the gas sample to be analyzed is located in a cylindrical vessel, in the longitudinal axis of which a thin measuring wire heated to a working temperature is arranged. If the heat conductivity of the gas changes due to an external magnetic field, this brings about a change in the resistance of the measuring wire, which change can be determined with a measuring bridge.
Complex fresh gas mixtures, which contain in most cases a binary basic mixture of oxygen and nitrogen, laughing gas or xenon and one of the common inhalation anesthetics (for example, desflurane, sevoflurane, isoflurane, enflurane, and halothane), are at times used in anesthesia for respirating patients in medical technology. It is frequently necessary for monitoring the patient to also carry out a determination of the gas concentrations during the expiration phase of the patient. The gas mixture additionally contains carbon dioxide, water vapor and possibly other metabolites, for example, ethyl alcohol, methane and acetone, besides said gases, during the expiration phase. Concerning the relevant gas concentrations, interest is limited here mainly to oxygen, carbon dioxide and the anesthetic as well as the dynamic changes of these components over time. Therefore, there is a need for cost-effective measuring devices, which detect such gases at the required resolution and possibly free from cross sensitivities. Several independent sensors, which are optimized for the particular target gas, are usually used for this.
It is known, for instance, the use of multichannel infrared optical analyzers with heat radiation sources, which are used to measure infrared-active gases and are capable of analyzing a gas mixture at a plurality of wavelengths for their absorption properties. Based on the spectra, which are recorded at least partly, the individual gaseous components can then be determined in terms of their concentration if they have sufficient and specific IR absorptions. However, gases such as oxygen, nitrogen, helium and xenon cannot be detected with this method.
Furthermore, sensors based on infrared laser diodes, which are capable, based on the narrow-band emission characteristic, to resolve the likewise narrow absorption lines of oxygen are known. However, a minimum absorption length, which leads to an unfavorable size of the sensor, is necessary because of the small absorption cross sections to carry out a concentration measurement with sufficient accuracy with this method. Moreover, interactions occur between the gases involved, which may at times require a correction of the measured O2 concentration values. Furthermore, this method is not suitable for the direct determination of the other gas components. Finally, this method, like the above-mentioned methods as well, is relatively expensive because of the high-quality optical components, especially because the laser diodes used have aging effects, which limit their service life.
Even though electrochemical sensors represent a cost-effective alternative to the IR optical methods, they meaningfully permit only a measurement of the oxygen concentration and—with great restrictions—of the carbon dioxide concentration. Anesthetic gases and noble gases cannot be measured in this manner.
In fixed electrolyte sensors, for example, those known from DE 20 2004 015 400 U1, a solid, for example, zirconium dioxide, assumes the task of an ion conductor. Thus, even though such sensors do have primarily a good selectivity for oxygen, they bring about decomposition processes in medical gas mixtures under certain circumstances because of the high operating temperature that is necessary to make ion conduction possible. The halogenated hydrocarbons commonly used in anesthesia are, in particular, no longer stable at operating temperatures of about 600° C. and at times produce highly toxic reaction products. In addition, laughing gas, which is also used in anesthesia, tends to decompose into nitrogen and oxygen at temperatures beginning from 400° C. and toxic nitrogen oxides may be formed as well. The oxygen released in this process will then lead to a falsely elevated concentration display. Oxygen concentrations can be reasonably measured with this principle of measurement in nitrogen/oxygen mixtures only. Other gases do not lend themselves to the analysis. However, this method is capable of detecting the flow parameter, which is likewise important, if the sensor is used in the mainstream.
Gas sensors based on heat conductivity are known from the literature, which operate either with heated metal wires or with resistive heating structures, which are applied to the membranes of microstructured silicon elements. The fact that the excess temperature of the wire or of the microstructured heating structure becomes established at a given electric heating energy as a function of the heat conduction properties of the carrier structure and of the gases surrounding the heating means is utilized in these sensors. The concentration ratios of binary gas mixtures can be unambiguously determined with such structures if the components of these mixtures have sufficiently different specific heat conductivities. Gas mixtures containing more than two components cannot be measured with this method. In particular, nitrogen/oxygen mixtures with additions of, for example, water vapor or CO2 cannot be meaningfully analyzed with this because of the similar specific heat conductivities of O2 and N2.
A gas sensor based on heat conduction, which utilizes the fact that the heat conductivity values of gases have certain temperature dependences, whose extent depends on the molecular structure of the gas in question, is known from EP 0 285 833 A2. It is proposed in that document that the gas sample to be analyzed at different measuring temperatures one after another and the concentrations of the different gases be inferred from the heat conductivity values measured at different temperatures. Mixtures containing three or more components can thus be analyzed, in principle. However, the requirement for this is a linear independence of the measured data sets, which is guaranteed in the normal case to a limited extent only. In addition, the sequential measurement in time presupposes a stable composition of the gas mixture at least for the duration of the analysis. The additional pneumatic means necessary for this make such a sensor expensive and adversely affect the overall size. Selective measurement of the oxygen concentration is not possible in this manner.
Documents DE 100 37 380 A1, DE 102 51 130 A1 and DE 102 41 244 C1 describe means that utilize the magnetic field-dependent heat conductivity of the oxygen component in gas mixtures for the concentration determination. The magnetic flux density is cyclically varied in these means in the measuring gap of an electromagnet and the heat conductivity of the gas mixture, which varies in the process, is detected with a heat conductivity measuring chip, which is likewise located in the measuring gap. The measuring chip has a heating means for this on a microstructured membrane, with which heating means part of the membrane is brought to a certain excess temperature, and a temperature-measuring unit, which is designed, for example, as a thermocouple (thermopile) and with which this temperature can be determined. In the presence of a paramagnetic gas, for example, oxygen, the specific heat conductivity of the oxygen component in the gas mixture changes due to cyclic modulation of the magnetic field, and this change will in turn lead to a variation of the measured temperature value, which can be determined, among other things, with a lock-in method. Since the magnitude of the temperature variations is also affected by the heat conduction properties of the other gases of the mixture, certain nonlinearities arise in the sensor characteristic, which depend on the nature of the gas components present.